Conventionally, a hydraulic circuit of an automatic transmission is provided with various control valves, such as a pressure control valve for controlling pressure, a flow control valve for controlling flow rate, and a direction control valve for controlling flow direction, for example, and by manipulating these control valves, oil is supplied to a hydraulic servo, thereby engaging a clutch, a brake, or the like, to change gear ratio (shift speed).
Each control valve includes a sleeve, a spool mounted so as to be freely slidable within the sleeve, a spring for urging the spool in one direction, and if necessary, a solenoid may also be provided for selectively moving the spool. The sleeve is formed with ports such as an inlet port for supplying oil to the control valve and an outlet port for discharging the oil from the control valve, and, when incorporated into a valve body, flow passages within in the valve body are in communication with each of the ports.
When foreign matter such as iron powder infiltrates a gap between the sleeve and the spool, it may cause the control valve to lock or damage the control valve. Therefore, to cover the ports, a strainer or filter structure is attached to the exterior of a strainer attachment portion of the sleeve, and the ports are sealed by tightly fitting the inner peripheral surface of the strainer over the outer peripheral surface of the strainer attachment portion, whereby foreign matter is prevented from infiltrating the valve. See, for example, Japanese Patent Application Publication No. JP-A-2001-41349 and Japanese Examined Patent Publication No. 3324140.